Not able to Assign Supplier in Carry out Sourcing in SRM Portal

Hi

Could you please tell anyone , the Purchaser have all the authorizations Operational Purchaser with sap_all in SRM & Portal roles , when he goes the Sourcing for carry out sourcing, unable to assign the suppler, since the Assign supplier button disabled. I have maintained the Roles in Attribuites as well ST_PURCHASER..